The electric machine is in particular a rotating electric machine such as a synchronous generator to be connected to a gas or steam turbine (turbogenerator) or a synchronous generator to be connected to a hydro turbine (hydro generator) or an asynchronous generator or a synchronous or asynchronous electric motor or also other types of electric machines.
In the technical field of generators, especially hydrogenerators, a rotor rotates within a stator with a certain speed. The mechanical system is designed for best operation in certain speed ranges. Further, the excess of a critical speed of the rotor is to be avoided. The critical speed is a quantity depending on several influences and is thus a unique quantity for each rotor. Furthermore, the critical speed can change over time and is not a constant quantity. The critical speed is a machine data which is of interest for the operator. The document JPH 0991044 describes a calculation method for calculating the critical speed of a rotor. Based on a mere mathematical approach in this document the eigenfrequency of the respective rotor is calculated from a variety of quantities as tool weight, static load, bending moment, rotor material, shear force; and further the critical speed of the rotor is determined from this calculated eigenfrequency.